lib: add const iteration & find to typesafe lists

lib: add const iteration & find to typesafe lists

Based on work originally by Mark Stapp <mjs@voltanet.io>.

Make it possible to iterate the typesafe lists in a const
context, as well as find items from them.

Signed-off-by: Mark Stapp <mjs@voltanet.io>
[above signoff was for the original version before modification]
Signed-off-by: David Lamparter <equinox@opensourcerouting.org>